Particle counters and sensors are used to detect light scattered by particles entrained in a stream of fluid, e.g., in an air stream. Such counters and sensors draw air (with entrained particles) from a room, for example, and flow such air along a tube and through an illuminated sensor "view volume" to obtain information about the number and size of such particles. Such information results from an analysis of the very small amounts of light reflectively "scattered" by the particle as it moves through the view volume.
Some types of sensors flow such air along an enclosed transparent tube; others "project" the air and accompanying particles at a particular flow rate (often measured in cubic feet per minute) from one tube across an open space to another tube. In sensors of the latter type, there is no tube wall (however transparent such wall may be) to impair light scattering and collecting. In other words, the particle is briefly illuminated by a very-small-diameter light beam is it "flies" through an open space.
Among other uses, particle counters incorporating particle sensors are used to obtain a measure of air quality by providing information as to the number and size of particles present in some specified volume of air, e.g., a cubic meter of air. Even work environments which appear to human observation to be clean--business offices, manufacturing facilities and the like--are likely to have substantial numbers of microscopic airborne particles. While such particles are not usually troublesome to the human occupants, they can create substantial problems in certain types of manufacturing operations.
For example, semiconductors and integrated chips are made in what are known as "clean rooms," the air in which is very well filtered. In fact, clean rooms are usually very slightly pressurized using extremely clean air so that particle-bearing air from the surrounding environs does not seep in. And the trend in the semiconductor and integrated chip manufacturing industry is toward progressively smaller products.
A small foreign particle which migrates into such a product during manufacture can cause premature failure or outright product rejection even before it is shipped to a customer. This continuing "miniaturization" requires corresponding improvements in clean-room environments (and in the related measuring instruments) to help assure that the number and size of airborne particles are reduced below previously-acceptable levels. While known particle counters and sensors have been generally acceptable for their intended purpose, certain disadvantages exist.
A disadvantage of known particle sensors involves the air passage, usually circular, along which air and entrained particles flow. In particular, such passage has a very small cross-sectional area. As a result, the pressure differential between the ends of the passage (sometimes referred to as the "pressure drop" across the passage) is quite high. It is not unusual to encounter a pressure drop in the range of 25-70 inches of water at a flow rate of about one cubic foot per minute (CFM). In the field of particle sensors, a pressure drop of 25-70 inches of water at that air flow rate is typical.
(Parenthetically, measuring pressure in inches of water is common. An analogy is found in older style blood pressure measuring devices which include a column of mercury contained in and visible through a graduated glass tube. Blood pressure is measured in "millimeters of mercury" and in such older style devices, blood pressure was equal to the column height. Blood pressure is still measured in millimeters of mercury but a different type of gauge is used to make the measurement.)
Because of the typical pressure drop along the very-small-area air flow passage, known sensors require a motor-driven positive displacement vacuum pump, usually of the diaphragm or vane type, to create enough vacuum to overcome such pressure drop. The necessary electric drive motor and vacuum pump are likely to be relatively heavy. And the motor requires outlet-sourced power; battery power is not practical because of the relatively large amount of power consumed. And because such a sensor requires an electrical cord and plug, it is not so readily moved from site to site, especially remote sites.
While the pressure drop along the air flow passage can be reduced by increasing the passage cross-sectional area, there is another design constraint which militates against that approach. To help assure accuracy in particle sensing and counting, all (or substantially all) of the air-entrained particles flowing along the passage must pass through the beam of light. Usually, the "flight path" of particles is perpendicular to such beam. However, the light beam is preferably sharply focused and its diameter is very small, e.g., less than about 0.1 inch. Therefore, the diameter of the air flow passage cannot be appreciably larger than that of the light beam and still assure that most or all of the particles will pass through the light beam and be detected.
The invention addresses these seemingly intractable difficulties and inconsistent design parameters in a unique and imaginative way.